Sometimes, you just can’t get the part quality that you want. For whatever reason, your machine just is not capable. Whether it is because your overhangs cause you to have to use a lot of supports which leave ugly marks on the surface, or your machine just doesn’t have the resolution to leave a good finish. There are a few things you can do.

• Tune your machine. Make sure that 25mm commanded in the gcode results in 25mm of material printed.
• Split your model into pieces. I print a lot of figurines for my kids. The hands and arms prove to be particularly difficult, as they need to be supported and they are small. When I cut the arms off and print them separately, I have much better results, then I just glue the arms onto the model.
• Primer/paint. A little paint covers a multitude of imperfections.